Marriage is profound, as Paul tells us in Ephesians 5:32. It is a mystery. Two become one. The depths of what this is really about transcend the definitions of any government. It refers to Christ and the church.

John Piper writes,

Leaving parents and holding fast to a wife, forming a new one-flesh union, is meant from the beginning to display this new covenant — Christ's leaving his Father and taking the church as his bride, at the cost of his life, and holding fast to her in a one-spirit union forever. (Momentary Marriage, 30).

And while sex itself does not make marriage, it certainly is meant to be a sign of marital union. It's a reaffirmation of oneness, and a celebration of the covenant that makes it so. So if this is what sex means, and marriage is "designed by God to reflect and display the spiritual union between Christ and the church," then sex is profound.

God created human beings in his image—male and female he created them, with capacities for intense sexual pleasure, and with a calling to commitment in marriage and continence in singleness [There are unique ways of knowing God through sexual continence in singleness and unique ways of knowing God through sexual intimacy in marriage].

And his goal in creating human beings with personhood and passion was to make sure that there would be sexual language and sexual images that would point to the promises and the pleasures of God’s relationship to his people and our relationship to him. In other words, the ultimate reason (not the only one) why we are sexual is to make God more deeply knowable. (Sex and the Supremacy of Christ, Part 1)